Iran appears to be gripped in a debate over how to respond to an international offer to stop its sensitive nuclear work, a senior US official said Monday.

Iran is sending mixed messages to a package of incentives from Germany and permanent UN Security Council members Britain, China, France, Russia and the United States, State Department spokesman Sean McCormack told reporters.

It appears that "from ...the varying public responses from various parts of the Iranian government that there is clearly a debate, or at least a discussion, going on within the Iranian government on how to respond," he said.

McCormack said that William Burns, a senior State Department official, would hold a conference call later Monday with his counterparts from the so-called P5-plus-1 to discuss "the current state of play" with Iran.

There has been considerable speculation in recent days that Tehran was softening its tone on the nuclear standoff.

However, European Union foreign policy chief Javier Solana said in Paris on Monday that he was not too optimistic about prospects for a breakthrough with Iran on its suspect nuclear program.

Solana said he hoped to meet soon with Iran's chief nuclear negotiator Saeed Jalili after Tehran on Friday delivered its response to the P5-plus-1 offer to halt uranium enrichment.

"It's difficult," he said, describing the response from Tehran as a "complicated and difficult letter that must be thoroughly analyzed."

In Tehran, President Mahmoud Ahmadinejad insisted Iran would not give up its right to a nuclear program and dismissed calls for Tehran to suspend uranium enrichment as a "repetitive scenario."

In an effort to persuade Iran to suspend uranium enrichment, the six powers have reworked an offer of technology and trade incentives originally made in 2006. Solana presented the offer on June 14 in Tehran.

Iran on Saturday offered to negotiate on its nuclear drive but without a freeze on uranium enrichment, in its first comments since responding to the package.

Iran insists its program to develop nuclear energy is peaceful, but Western powers fear Tehran is seeking to develop atomic weapons.

The West has warned that Tehran faces more sanctions from the United Nations and European Union if it does not freeze enrichment.